1. Field of the Invention
The present invention relates generally to the field of robotics and more particularly to end-effectors used to grasp objects.
2. Description of the Prior Art
One goal of robotics is to develop robots that can better be used in unstructured environments. Towards this goal, it is necessary to improve robotic grasping so that robots are able to more securely grasp a wider variety of objects that differ in size, shape and weight. An improved end-effector, or hand, for robotic grasping should be simple, robust, and reliable.
Robots used in industrial environments have end-effectors designed specifically for the parts to be grasped. Such special purpose end-effectors are ideal for grasping a single type of part repeatedly and over a long production run. Robots that are able to handle several part types typically employ a turret with multiple end-effectors, or a tool changer is used. These alternatives are feasible so long as the number of different parts involved is small.
In contrast, a robot to be used in an unstructured environment would ideally have an end-effector that can securely grasp a wide variety of objects differing considerably in their size, shape, and weight. Such end-effectors must be able to adapt to the object to be grasped and are generally referred to as robotic “hands.”
The many designs for robotic hands can be broadly classified into two groups. The hands of the first group have simple linkages and are limited in their adaptability, while the hands of the second group employ complex linkages to provide greater adaptability. An early design of the first kind is described in a paper by F. Skinner, “Multiple Prehension Hands for Assembly Robots,” Proc. 5th International Symposium on Industrial Robotics, Chicago, 1975, and in U.S. Pat. Nos. 3,866,966 and 3,901,547. Another early hand of the first kind is described in a paper by A. Rovetta, “On Specific Problems of Design of Multi-Purpose Mechanical Hand Industrial Robots,” Proc 7th ISIR, Tokyo, 1977. A later design with simple linkages is described in a paper by N. Ulrich, R. Paul, and R. Bajcsy, “A Medium-Complexity Complaint End Effector, Proc. IEEE International Conference on Robotics and Automation, Philadelphia, 1988, and in U.S. Pat. Nos. 4,957,320 and 5,501,498. Another design with simple linkages is described in a paper by K. Mirza and D. E. Orin, “Force Distribution for Power Grasps in the Digits System,” Symposium Theory and Practice of Robots and Manipulators, 1990. U.S. Pat. Nos. 4,792,338 and 5,108,140 provide still further robotic hands with simple linkages.
The hands of the second group have complex linkages to offer greater adaptability. These include a hand described in a paper by K. Salisbury, Robot Hands and the Mechanics of Manipulation, MIT Press, 1985, the “Utah/MIT hand” described in a paper by S. Jacobson, “Design of the Utah/MIT Dexterous Hand,” Proc. of the IEEE International Conference on Robotics and Automation, 1986, a hand described in a paper by G. Vassura and A. Bicchi, “Whole-Hand Manipulation: Design of an Articulated Hand Exploiting All Its Parts to Increase Dexterity,” in Robots and Biological Systems, ser. NATO-ASI, New York, Springer-Verlag, 1989, and the “DLR” hand described in a paper by G. Hirzinger, “Torque-controlled Light Weight Arms and Articulated Hands,” in Siciliano (ed), Experimental Robotics VIII, Springer, 2003. U.S. Pat. Nos. 5,172,951, 5,447,403, 5,588,688, 6,244,644, and 6,517,132 and U.S. Patent Application 2003/0090115 also disclose further robotic hands with complex linkages.
Hands with great adaptability employ fingers with multiple active joints per finger. To position a tip of a finger to an arbitrary position in 3-dimensions, at least three degrees of freedom are required. As each active joint can be used to control one degree of freedom, most adaptable robotic hands have three or more fingers, each with three or more active joints. For example, the MIT/Utah hand has a total of 16 active joints. The hand described in U.S. Pat. No. 6,244,644 issued to Lovchik has two fingers and one thumb each with three joints plus three other active joints, for a total of 12 active joints. The DLR hand has four fingers with a total of 13 active joints. The use of a large number of active joints, however, causes hands to be mechanically complex as each active joint can include an actuator, a controller, and means for speed reduction and power transmission. The large number of components combined with stringent space constraints make these designs costly, difficult to manufacture, and expensive to maintain.
Some robotic hands can form grasps by using a palm in addition to the fingers. As explained in more detail below, grasps that utilize a palm are typically more robust than those that only use fingers. Several of the hands described above utilize a palm to form grasps. Skinner, Ulrich, Mirza, Vassura, Hirzinger, U.S. Pat. Nos. 3,866,966, 5,108,140, 5,172,951, 5,501,498, 5,588,688 and 6,517,132, and U.S. Patent Application 2003/0090115 each have a fixed palm. Rovetta has a spring-loaded palm, but one that is not actuated. U.S. Pat. Nos. 4,792,338, and 6,244,644 each have an actuated palm, but one in which the palm is coupled to one or more of the fingers and in which the palm motion is relatively small. In each of these designs the ability of the palm to participate in a wide range of desirable grasps is limited.
Hence, there is a need for a robotic hand having a palm that is moveable over a wide range and can be actuated independently of the fingers.